It is well known in the case of optical measuring instruments to use dual optical beams comprising a modulation beam and a reference beam to distinguish between changes due to the test parameter and any other changes which tend to obfuscate the parameter induced signal. The modulated signal beam is subject to the test parameter as well as undesirable influences such as variations in the light source intensity, coupling variations or variable temperature response of detectors, and the reference beam is subject to all the same influences except the test parameter. By comparing the two beams the changes due to the test parameter are readily isolated.
Fiber optic microbend sensors have incorporated the dual beam principle by using one optical fiber subject to microbending to obtain a modulation signal carrying the parameter information along with unwanted fluctuations and a separate reference optical fiber operating at the same wavelength but not subject to microbending to obtain a reference signal carrying only the unwanted fluctuations so that a comparison of the two outputs allows the true microbend signal value to be determined. Since separate optical paths are required, the two signals are not necessarily subject to the same disturbances.
It is desirable to eliminate the reference fiber since it introduces expense and complexity to the sensor, yet it is necessary to compensate for the unwanted fluctuations. Prior to this invention it had been believed that different wavelengths in an optical fiber subject to microbending were similarly affected by microbending. In particular, wavelengths of comparable value were reported to experience only slight dispersion of microbending losses and would not be useful for the single fiber microbend sensor disclosed herein. See, for example, D. Marcuse, "Microdeformation Losses of Single-mode Fibers," Appl. Opt., Vol, 23, 1082-1091, 1984. That report concerned random microbending. Similar results were known for periodic microbending in multi-mode fibers.